The repair or augmentation of soft tissue defects or contour abnormalities caused by facial defects, acne, surgical scarring or aging has proven to be very difficult. A number of materials have been used to correct soft tissue defects with varying degrees of success, but currently no material appears to be completely safe and effective. In the past, small amounts of liquid silicone were used to correct minor soft tissue defects where minimal mechanical stress was present at the recipient site. Unfortunately, liquid silicone from these injections appears to migrate to distant body parts and causes a variety of physiological and clinical problems. In response to these problems and the misuse of liquid silicone, the FDA has prohibited the use of liquid silicone in humans.
In the 1970's, reconstituted injectable bovine collagen became available and appeared to be an effective treatment for soft tissue defects. However, over time, the benefits of the collagen treatment have proven to be short-lived; the collagen reabsorbs in two to three months. Additionally, safety measures must be employed with this material to avoid allergic reactions to the bovine proteins in the collagen. To solve these shortcomings, crosslinked collagen has been introduced to extend the effect of treatments to approximately six (6) months. However, allergic reactions still occur with the crosslinked collagen material and frequent readministration of the crosslinked material is still required.
Recently, several authors have described new materials that may be used for soft tissue repair or augmentation such as biocompatible ceramic particles in aqueous gels, thermoplastic materials, thermosetting materials and lactic acid based polymer blends that avoid some of the problems previously experienced with collagen and liquid silicone.
Injectable implants of biocompatible ceramic particles in aqueous gels were first proposed by Wallace et al. in U.S. Pat. No. 5,204,382. The implants consisted of ceramic particles of calcium phosphate from a nonbiological source, mixed with an aqueous gel carrier in a viscous polymer (such as polyethylene glycol, hyaluronic acid, poly(hydroxyethyl methacrylate) and collagen). Although these materials are generally nontoxic, there appears to be risks associated with the use of nonabsorbable particulate materials related to the migration of these particles to distance sites in the body.
Thermoplastic and thermosetting defect fillers were originally described by Dunn et al. in U.S. Pat. Nos. 4,938,763, 5,278,201 and 5,278,202. In these patents, Dunn proposes the use of both a thermoplastic material with a solvent and a thermosetting material with a curing agent to form solid implants in situ. Although the biodegradable materials Dunn suggests for use as thermoplastics appear acceptable, the solvents necessary to dissolve them for injection into tissue appear to be less than acceptable. Additionally, Dunn's thermoplastic and thermosetting materials have limited utility in filling soft tissue because they solidify. Similar commercially available materials exhibit ultimate yield stresses of approximately 10,000 psi; in comparison, human skin exhibits ultimate yield stresses of from 500 to 2,000 psi. Therefore, due to palpability concerns, the thermoplastic and thermosetting materials that Dunn proposed appear to be too hard for use in soft tissue augmentation or repair and especially in dermal augmentation or repair.
Soft tissue repair or augmentation has also been proposed using lactic acid based polymer blends of amorphous oligomers with crystalline oligomers or polymers (Buchholz et al. U.S. Pat. No. 4,235,312 A1). Buchholz's blends were developed to provide a pasty to waxy material which could be used as an absorbable implant to replace the brittle copolymers of lactic acid and glycolic acid already described for use as bone waxes. However, these blends do not appear to be suitable for use as injectable soft tissue defect fillers, because they are too viscous to be injected through a needle which significantly limits the utility of these blends. Furthermore, the low molecular weight liquid oligomers described by Buchholz are slightly soluble in body fluids, which means that these oligomers will quickly diffuse out of the site of implantation to other areas of the body.
In view of the deficiencies of the soft tissue augmentation materials previously considered, it is evident that new soft tissue augmentation materials need to be developed. Ideally, any new augmentation material would have several important characteristics not possessed by any one of the previously discussed materials. For example, any new augmentation material should be completely bioabsorbable to avoid the possibility of long term chronic irritation of tissues or migration of nonabsorbable materials over time to different areas of the body. The new augmentation materials should also provide soft tissue augmentation for at least six months to avoid frequent readministration of the augmentation material. Furthermore, new soft tissue augmentation materials should be easy to administer preferably by injection. Finally, the ideal soft tissue augmentation material would have the appropriate degree of pliability for the tissue into which the new material was being implanted to provide life like tissue augmentation. As discussed above, none of the currently available materials have all of these characteristics.
Therefore, it is an object of the present invention to provide a safe, injectable, long lasting, bioabsorbable, soft tissue repair and augmentation material.